1. Field of the Invention
This invention relates to a method of shaping at least two glass sheets to a complicated shape using gravity sag bending to produce a relatively simple prebent shape followed by blow forming to change the relatively simple prebent shape into a more complicated final shape. A typical shape produced by the present invention is found in the complicated window shapes for a Cessna 421 airplane. However, while the present invention was designed especially for producing an aircraft windshield, it is equally well suited for the production of other transparent articles.
Glass sheets have been shaped while heated to a deformation temperature by various methods. These include gravity sag bending in which one or more glass sheets are permitted to sag until their lower surface conforms to the upward facing surface of a mold of desired contour. Another known method involves press bending wherein a glass sheet is sandwiched in pressurized engagement between a pair of press bending molds of complementary contour. Usually, the molds are covered by a material that reduces the marking on the hot glass. Fiber glass cloth is a well-known material for this purpose. Both of the press bending molds between which glass is sandwiched during press bending may be rigidly formed. However, it is also known to shape glass sheets between a flexible member and a rigid member. It is also known to shape or reshape glass sheets by applying pressure through a fluid against a flexible member that presses a heat-softened glass sheet against a rigid mold having a shaping surface of the desired shape.
In gravity sag bending, glass sheets are shaped to conform either at their outlines only or to conform throughout their entire extent to the shape of an upward facing shaping surface of the gravity sag mold. When dirt particles are present on the upper shaping surface of a continuous shaping mold, the surface of the glass is marred. When a glass sheet is sag bent to conform to the shape of an outline mold, it is difficult to control the intermediate sag bounded by the marginal portion that conforms to the shape of the shaping surface of an outline mold.
When pressure is applied to help form a glass sheet, any deviations from surface smoothness on the press forming mold are replicated in the heat-softened surface of the glass sheet undergoing shaping. The presence of a parting material or cover for the press bending molds reduces the severity of surface marking due to pressure of a solid member against a heat-softened glass sheet surface. However, such surface markings are difficult to remove altogether.
When glass sheets are cradled between a flexible hammock of fiber glass cloth and the covered shaping surfaces of press bending molds, it is necessary to spring load the flexible fiber glass material so as to avoid marking the heat-softened glass sheet surfaces with wrinkles formed on the fiber glass material contacting the glass.
It has also been proposed to use pressure forming as a means of shaping glass sheets. When glass sheets are shaped by pressure of a fluid applied through a flexible member against one surface of a glass sheet to bring the opposite surface of a glass sheet against a shaping surface of a solid mold, it is still difficult to avoid entirely replicating the defects on the solid mold on the surface of the sheet that contacts or is brought into pressurized engagement against the pressing mold.
Glass sheets have also been shaped by blow forming. However, during blow forming, the thickness of the glass sheet that results from such shaping is nonuniform and the degree of nonuniformity varies considerably with the size of the sheet and the degree of curvature to which the sheet is to be shaped.
2. Description of the Prior Art
U.S. Pat. No. 2,123,552 to Helwig discloses the blow forming of plastic sheeting against a so-called negative mold having a three dimensional curved surface allegedly without marring the surface of the sheet undergoing shaping. The plastic sheet is blown against a mold having a convex shaping surface by the application of fluid pressure. When the fluid pressure exceeds a predetermined amount, which is associated with the completion of the shaping, the pressure lifts the mold. This actuates a solenoid valve circuit which stops the application of pressure so as to produce a shaped plastic sheet that does not come into contact with the mold shaping surface and which, as a consequence, has optically clear surfaces.
U.S. Pat. No. 2,218,654 to Paddock discloses a two-step method for shaping glass sheets to deep bends. The first step is an over bend where the glass, while supported on pipes between its ends and on additional pipes at its ends is heated to a glass softening temperature. The end supporting pipes are removed to permit the glass sheet to sag into a convexly elevated shape. The glass sheet is then turned upside-down and shaped additionally by gravity sagging through the application of heat while the upside-down glass sheet is supported over a bending mold having a shaping surface of concave elevation.
U.S. Pat. No. 2,377,849 to Binkert and Jendrisak discloses a process of bending a stack of glass sheets by prebending them by gravity to conform to the shape of a first mold followed by shaping the glass sheets by a combination of gravity and suction to conform to a second mold of compound curvature.
U.S. Pat. No. 2,518,896 to Jendrisak discloses a technique of bending a pair of glass sheets supported in an oblique orientation over an outline shaping mold. A pivotable frame member of complemental shape rests on the upper shaping surface of the glass sheet when the latter is heat-softened to pressure form the glass to a shape conforming to that of the upwardly facing outline shaping surface of the mold beneath the glass sheet.
U.S. Pat. No. 3,473,909 to Bennett, Blanding and Hausheer discloses a method of correcting distortions in the shaping of glass articles such as spherical glass lenses or television tube or cathode ray tube face plates by supporting a sheet that has become distorted during its original shaping by applying pressure through a stretched membrane against the upper surface of the glass article to conform the shape of its lower surface to an upward facing surface of the mold that supports the lower surface of the glass article when pressure is applied through the flexible stretched membrane.
U.S. Pat. No. 3,484,225 to Bognar reforms the inner surface of a cathode ray tube having phosphors applied thereto by heating a deformed or distorted cathode ray tube in an oven to a temperature equal to the softening point at its outer layer and equal to its annealing point at its inner layer, applying pressure through a flexible member and a tautly held heat shield member against the upper surface of the cathode ray tube while the lower surface of the cathode ray tube is pressed against the upper shaping surface of a heated mold.
U.S. Pat. No. 3,600,150 to Rougeux uses a flexible hammock that is spring loaded to support a glass sheet between upper and lower shaping surfaces of complementary curvature whose glass facing surfaces are covered. This patent also discloses the concept of enlosing a vertically suspended glass sheet in a flexible fiber glass sling and shaping the heat-softened glass sheet together with the sling between a pair of press forming members.
In forming complicated bends to conform to the shape required for certain aircraft windshields, or to form glass shaping plates which are used to press polish and simultaneously laminate plastic windows for aircraft of the type having an outer sheet of acrylic plastic or polycarbonate plastic, where it is desirable that the glass sheets used for such pressing be smoothly shaped and of exact confirmation in shape to one another, it is necessary to avoid any marring of the glass and that the glass be formed by a precision forming process at as close to the required shape throughout its entire extent as possible. It is also desirable to minimize the deviation in thickness that accompanies blow forming and still obtain shape conformity to desired complex shapes with minimum surface marking. This precision shaping must be performed at a minimum temperature possible using a low melting glass composition that devitrifies at a temperature slightly above its annealing range. Hence, it is necessary to develop a technique that combines the best features of several prior art glass sheet shaping methods in a novel combination of steps that results in shaping glass sheets to a precise compound bend without devitrifying the highly devitrifiable glass composition.